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DOE/EIA-TR-0565Distribution Category UC-950Drilling Sideways -- A Review of Horizontal WellTechnology and Its Domestic ApplicationApril 1993Energy Information AdministrationOffice of Oil and GasU.S.Department of EnergyWashington,DC 20585This report was prepared by the Energy Information Administration,the independent statistical and analytical agency within theDepartment of Energy.The information contained herein should not be construed as advocating or reflecting any policy position ofthe Department of Energy or any other organization.ContactsThis report was prepared by the Energy Information Administration,Office of Oil and Gas,under thegeneral direction of Diane W.Lique,Director of the Reserves and Natural Gas Division,Craig H.Cranston,Chief of the Reserves and Production Branch,and David F.Morehouse,Senior SupervisoryGeologist.Information regarding the content of this report may be obtained fromits author,Robert F.King (202)586-4787,or from David Morehouse (202)586-4853.iiDrilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information AdministrationPrefaceDrilling Sideways -- A Review of Horizontal Well Technology and Its Domestic Application is the secondOffice of Oil and Gas review of a cutting-edge upstream oil and gas industry technology and its currentand possible future impacts.The first such review,Three-Dimensional Seismology -- A New Perspective,authored by Robert Haar,appeared as a feature article in the December 1992 issues of the EnergyInformation Administration's Natural Gas Monthly and PetroleumSupply Monthly.Additional technologyreviews will be issued as warranted by new developments and lack in the extant literature of reviews ofa similar nature and breadth.The technology reviews are intended for use by a wide and varied audience of energy analysts andspecialists located in government,academia,and industry.They are syntheses based on an extensiveliterature search and direct consultations with developers and users of the technology.Each seeks tooutline,as concisely as possible,the involved basic principles,the current state of technologydevelopment,the current status of technology application,and the probable impacts of the technology.Where possible,the economics of technology appplication are explicitly addressed.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information AdministrationiiiContentsPageHighlights............................................................vii1.Introduction.......................................................1Definition and Immediate Technical Objective................................1Drilling Methods and Some Associated Hardware..............................1Some New Terminology...............................................3The Essential Economics of Horizontal Drilling...............................4The Cost Premium.................................................4Desired Compensating Benefits........................................42.History of Technology Development and Deployment.........................7Halting Steps.......................................................7Early Commercial Horizontal Wells.......................................7The Recent Growth of Commercial Horizontal Drilling..........................8Types of Horizontal Wells and Their Application Favorabilities....................8Short-radius Horizontal Wells.........................................8Medium-radius Horizontal Wells.......................................9Long-radius Horizontal Wells.........................................9Attainable Length of Horizontal Displacement................................9The Completion of Horizontal Wells for Production............................10Well Logging and Formation Testing....................................10Wall Cleanup and Well Stimulation.....................................10Open Hole Completion..............................................11Cased Completion.................................................11Casing.......................................................11Cementing....................................................11Packers......................................................12Perforations and Liners...........................................12Current Domestic Applications...........................................13"Chalk Formations"................................................13Other Applications.................................................15Source Rock Applications.........................................15Stratigraphic Trap Applications.....................................16Heterogeneous Reservoirs.........................................16Coalbed Methane Production.......................................16Boosting Recovery Factor.........................................17Fluid and Heat Injection Applications.................................17Multiple Objectives.............................................18Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information AdministrationvPage3.New Developments..................................................21Expected Growth of Horizontal Drilling....................................21Coiled Tubing and Horizontal Drilling.....................................21Slim Hole Horizontal Drilling...........................................23The Drilling of Multiple Laterals.........................................23A"Fire and Forget"Drilling System.......................................23Gas Research Wells..................................................244.Summary.........................................................25Tables1.Bakken Shale Horizontal Well Costs.......................................62.Giddings Field Horizontal Well Production,6-Month Average.....................143.Horizontal Oil Activity and Production,North Dakota...........................17Figure1.Greater Length of Producing Formation Exposed to the Wellbore in a HorizontalWell (A) Than in a Vertical Well (B)......................................2viDrilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information AdministrationHighlightsThe use of horizontal drilling technology in oil exploration,development,and production operations hasgrown rapidly over the past 5 years.This report reviews the technology,its history,and its currentdomestic application.It also considers related technologies that will increasingly affect horizontaldrilling's future.Horizontal drilling technology achieved commercial viability during the late 1980's.Its successfulemployment,particularly in the Bakken Shale of North Dakota and the Austin Chalk of Texas,hasencouraged testing of it in many domestic geographic regions and geologic situations.Of the three majorcategories of horizontal drilling,short-,medium-,and long-radius,the medium-radius well has been mostwidely used and productive.Achievable horizontal bore hole length grew rapidly as familiarity with thetechnique increased;horizontal displacements have now been extended to over 8,000 feet.Some wellshave featured multiple horizontal bores.Completion and production techniques have been modified forthe horizontal environment,with more change required as the well radius decreases;the specific geologicenvironment and production history of the reservoir also determine the completion methods employed.Most horizontal wells have targeted crude oil reservoirs.The commercial viability of horizontal wells forproduction of natural gas has not been well demonstrated yet,although some horizontal wells have beenused to produce coal seam gas.The Department of Energy has provided funding for several experimentalhorizontal gas wells.The technical objective of horizontal drilling is to expose significantly more reservoir rock to the well boresurface than can be achieved via drilling of a conventional vertical well.The desire to achieve thisobjective stems from the intended achievement of other,more important technical objectives that relateto specific physical characteristics of the target reservoir,and that provide economic benefits.Examplesof these technical objectives are the need to intersect multiple fracture systems within a reservoir and theneed to avoid unnecessarily premature water or gas intrusion that would interfere with oil production.Inboth examples,an economic benefit of horizontal drilling success is increased productivity of the reservoir.In the latter example,prolongation of the reservoir's commercial life is also an economic benefit.Domestic applications of horizontal drilling technology have included the drilling of fractured conventionalreservoirs,fractured source rocks,stratigraphic traps,heterogeneous reservoirs,coalbeds (to produce theirmethane content),older fields (to boost their recovery factors),and fluid and heat injection wells intendedto boost both production rates and recovery factors.Significant successes include many horizontal wellsdrilled into the fractured Austin Chalk of Texas'Giddings Field,which have produced at 2.5 to 7 timesthe rate of vertical wells,wells drilled into North Dakota's Bakken Shale,from which horizontal oilproduction increased from nothing in 1986 to account for 10 percent of the State's 1991 production,andwells drilled into Alaska's North Slope fields.An offset to the benefits provided by successful horizontal drilling is its higher cost.But the average costis going down.By 1990,the cost premium associated with horizontal wells had shrunk from the 300-percent level experienced with some early experimental wells to an annual average of 17 percent.Learning curves are apparent,as indicated by incurred costs,as new companies try horizontal drilling andas companies move to new target reservoirs.It is probable that the cost premium associated withhorizontal drilling will continue to decline,leading to its increased use.Two allied technologies arecurrently being adapted to horizontal drilling in the effort to reduce costs.They are the use of coiledtubing rather than conventional drill pipe for both drilling and completion operations and the use ofsmaller than conventional diameter (slim) holes.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administrationvii1.IntroductionThe application of horizontal drilling technology to the discovery and productive development of oilreserves has become a frequent,worldwide event over the past 5 years.This report focuses primarily ondomestic horizontal drilling applications,past and present,and on salient aspects of current and near-futurehorizontal drilling and completion technology.Definition and Immediate Technical ObjectiveA widely accepted definition of what qualifies as horizontal drilling has yet to be written.The followingcombines the essential components of two previously published definitions:1Horizontal drilling is the process of drilling and completing,for production,a well that beginsas a vertical or inclined linear bore which extends from the surface to a subsurface location justabove the target oil or gas reservoir called the"kickoff point,"then bears off on an arc tointersect the reservoir at the"entry point,"and,thereafter,continues at a near-horizontal attitudetangent to the arc,to substantially or entirely remain within the reservoir until the desired bottomhole location is reached.Most oil and gas reservoirs are much more extensive in their horizontal (areal) dimensions than in theirvertical (thickness) dimension.By drilling that portion of a well which intersects such a reservoir parallelto its plane of more extensive dimension,horizontal drilling's immediate technical objective is achieved.That objective is to expose significantly more reservoir rock to the wellbore surface than would be thecase with a conventional vertical well penetrating the reservoir perpendicular to its plane of more extensivedimension (Figure 1).The desire to attain this immediate technical objective is almost always motivatedby the intended achievement of more important objectives (such as avoidance of water production) relatedto specific physical characteristics of the target reservoir.Several examples of these are discussed lateron.Drilling Methods and Some Associated HardwareThe initial linear portion of a horizontal well,unless very short,is typically drilled using the same rotarydrilling technique that is used to drill most vertical wells,wherein the entire drill string is rotated at thesurface.The drill string minimally consists of many joints of steel alloy drill pipe,any drill collars(essentially,heavy cylinders) needed to provide downward force on the drill bit,and the drill bit itself.Depending on the intended radius of curvature and the hole diameter,the arc section of a horizontal wellmay be drilled either conventionally or by use of a drilling fluid-driven axial hydraulic motor or turbinemotor mounted downhole directly above the bit.In the latter instance,the drill pipe above the downholemotor is held rotationally stationary at the surface.The near-horizontal portions of a well are drilled usinga downhole motor in virtually all instances.1Joshi,S.D.,Horizontal Well Technology,PennWell Books,Tulsa,Oklahoma,p.1.;Shelkholeslami,B.A.,B.W.Schlottman,F.A.Seidel and D.M.Button,"Drilling and Production of Horizontal Wells in the Austin Chalk,"Journal of PetroleumTechnology,July 1991,p.773.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration1Source:Energy Information Administration,Office of Oil and Gas.Figure 1.Greater Length of Producing Formation Exposed to the Wellbore in a Horizontal Well(A) Than in a Vertical Well (B).It is possible to drill the arc section of the well bore because the apparently rigid drill pipe sections are,in fact,sufficiently flexible that each can be bent a distance off the initial axis without significant risk ofincurring a structural failure such as buckling or twisting off.The smaller the pipe diameter and the moreductile the steel alloy,the greater the deviation that can be achieved within a given drilled distance.Thatis,the smaller the arc's radius can be made,or the larger the arc's build angle2can be.Downhole instrument packages that telemeter various sensor readings to operators at the surface areincluded in the drill string near the bit,at least while drilling the arc and near-horizontal portions of thehole.Minimally,a sensor provides the subsurface location of the drill bit so that the hole's direction,asreflected in its azimuth and vertical angle relative to hole length and starting location,can be tightlycontrolled.Control of hole direction (steering) is accomplished through the employment of at least oneof the following:· a steerable downhole motor· various"bent subs"· pipe stabilizers."Bent subs"are short sub-assemblies that,when placed in the drill string above the bit and motor,introduce small angular deviations into the string.Pipe stabilizers are short sub-assemblies that are widerthan the drill pipe,but usually slightly narrower than the bit diameter.They are included at intervalsalong the drill string wherever precise lateral positioning of the pipe in the hole is needed.If they aresymmetrical,they simply center the pipe within the drilled hole.If asymmetrical,they will induce a small2Domestically,build angle is measured in degrees of angular change per 100 feet drilled.2Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administrationangle between the pipe and the hole wall.All of these devices can be obtained in lower cost versionswhere the induced angular deviation can only be adjusted at the surface,or in higher cost versions thatcan be remotely adjusted while they are downhole.The additional cost of remote control capability may,in many instances,be outweighed by time-related savings,due to a substantial reduction of the numberof trips3needed,many of which would be made for the sole purpose of direction adjustment.Additional downhole sensors can be,and often are,optionally included in the drill string.They mayprovide information on the downhole environment (for example,bottom hole temperature and pressure,weight on the bit,bit rotation speed,and rotational torque).They may also provide any of severalmeasures of the physical characteristics of the surrounding rock and its fluid content,similar to thoseobtained via conventional wireline well logging methods,but in this case obtained in real time whiledrilling ahead.The downhole instrument package,whatever its composition,is referred to as ameasurement-while-drilling (MWD) package.Some New TerminologyThe advent of commercial horizontal drilling has inevitably added newabbreviated terms to the"oil patch"lexicon.Expanding beyond the"old standard"vertical well statistic TD,denoting the total depth of a holeas measured along the length of the bore,the following terms,which appear frequently elsewhere in thisarticle,are now widely used to quantify the results of horizontal drilling:AbbreviatedTerm:Stands for:Denotes:TVDTotal Vertical DepthTotal depth reached as measuredalong a line drawn to the bottom ofthe hole that is also perpendicular tothe earth's surface.MSDMeasured DepthTotal distance drilled as measuredalong the well bore.Note that in avertical hole,MSD would equal TD.HDHorizontal DisplacementTotal distance drilled along the quasi-horizontal portion of the well bore.3A"trip"encompasses removal of the entire drill string from the hole,usually for the purposes of adjustment or change ofhardware,followed by reinsertion.A typical trip to a depth of several thousand feet can consume several hours,during whichtime no forward progress is made while the operating and rig rental costs continue unabated.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration3The Essential Economics of Horizontal DrillingThe Cost PremiumThe achievement of desired technical objectives via horizontal drilling comes at a price:a horizontal wellcan be anywhere from 25 percent to 300 percent more costly to drill and complete for production thanwould be a vertical well directed to the same target horizon.Some costs presumably typical of ahorizontal well drilled in the Bakken Formation of North Dakota (a Mississippian shale) were reportedin the form of an example authorization for expenditure by the Oil and Gas Investor,(Table 1).Theexample well would have a measured depth (MSD) of 13,600 feet and a horizontal displacement (HD)of 2,500 feet.Petroleum Engineer International (PEI) reported in November 1990 that,according to a PEI survey,horizontal wells drilled in the U.S.during the prior year had averaged slightly over $1 million per wellto drill,plus an additional $140,000 per well to complete for production.The average cost per foot ofhorizontal displacement was $475 nationally,and $360 for horizontal wells drilled into the UpperCretaceous Austin Chalk Formation of Texas4,while some experienced companies got close to $300/footin 1990.5The cost difference,which in part implies that horizontal wells targeted at other than the AustinChalk were more expensive,reflects a combination of sometimes radically different drilling conditions.At this early stage of technology application,each new type of target begets a"learning curve"which mustbe followed to develop optimal drilling and completion techniques for that target.Costs of successivewells tend to fall as more is learned and technique is optimized on the basis of that knowledge.Also,the industry's Joint Association Survey on 1990 Drilling Costs6reported that average horizontaldrilling cost per foot was $88.16 as compared to $75.40 for wells not drilled horizontally,a 17-percentcost premium.Total expenditures on horizontal drilling reached $662 million in 1990,representing6 percent of the total drilling expenditure of $10.937 billion.Desired Compensating BenefitsEven when drilling technique has been optimized for a target,the expected financial benefits of horizontaldrilling must at least offset the increased well costs before such a project will be undertaken.In successfulhorizontal drilling applications,the"offset or better"happens due to the occurrence of one or more of anumber of factors.First,operators often are able to develop a reservoir with a sufficiently smaller number of horizontal wells,since each well can drain a larger rock volume about its bore than a vertical well could.When this is thecase,per well proved reserves are higher than for a vertical well.An added advantage relative to theenvironmental costs or land use problems that may pertain in some situations is that the aggregate surface"footprint"of an oil or gas recovery operation can be reduced by use of horizontal wells.4Steven D.Moore,"Horizontal Drilling Activity Booms,"Petroleum Engineer International,(November 1990),pp.15-16.5Steven D.Moore,"The Horizontal Approach,"Petroleum Engineer International,(November 1990),p.6.6Finance,Accounting and Statistics Department,American Petroleum Institute,Joint Association Survey on 1990 DrillingCosts,(November 1991),pp.4-5.4Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information AdministrationSecond,a horizontal well may produce at rates several times greater than a vertical well,due to theincreased wellbore surface area within the producing interval.For example,in the Austin Chalk reservoirof Texas'Giddings Field,under equal pressure conditions,horizontal completions of 500 to 2,200 footHD produce at initial rates 2 to 7 times higher than vertical completions.7Chairman Robert Hauptfuhrerof Oryx Energy Co.noted that"Our costs in the [Austin] chalk now are 50 percent more than a verticalwell,but we have three to five or more times the daily production and reserves than a vertical well."8A faster producing rate translates financially to a higher rate of return on the horizontal project than wouldbe achieved by a vertical project.Third,use of a horizontal well may preclude or significantly delay the onset of production problems(interferences) that engender low production rates,low recovery efficiencies,and/or premature wellabandonment,reducing or even eliminating,as a result of their occurrence,return on investment and totalreturn.7B.A.Shelkholeslami and others,"Drilling and Production Aspects of Horizontal Wells in the Austin Chalk,"Journal ofPetroleum Technology,(July 1991),SPE Paper Number,p.779.8"Oryx's Hauptfuhrer:Big increase due in U.S.horizontal drilling,"Oil & Gas Journal,(January 15,1990),p.28.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration5Source:Sandra Johnson,"Bakken Shale,"Oil and Gas Investor,(June 1990),p.36,Table 1.Bakken Shale Horizontal Well Costs(1990 Dollars)Cost Category Drill & Test Complete TotalSurvey & Permits 3,900 0 3,900Building Road & Location 23,850 2,500 26,350Footage Contract 145,085 0 145,085Day Work Contract 101,200 0 101,200Rig For Completion 0 20,500 20,500Drill Bits 37,020 450 37,470Rental Equipment 60,325 16,700 77,025Labor & Travel 45,300 28,200 73,500Trucking & Hauling 23,650 9,500 33,150Power,Fuel & Water 3,300 0 3,300Mud & Chemicals 97,000 0 97,000Drill Pipe 16,930 0 16,930Mud Logging 14,550 0 14,550Logs 12,600 11,000 23,600Bottom Hole Pressure Test 0 5,000 5,000Directional Services 200,000 0 200,000Engineering and Geology 12,900 2,000 14,900Cementing Surface Casing 14,000 0 14,000Cementing Production Casing 0 53,400 53,400Cleaning Location 4,600 500 5,100Environment & Safety Eqt 7,200 0 7,200Misc.Material & Service 16,825 4,200 21,025Total Intangibles 840,235 153,950 994,185Surface Casing 9 5/8"47,320 0 47,320Production Casing 5 1/2"0 198,435 198,435Tubing 2 7/8"0 42,840 42,840Christmas Tree & Tubing Head 2,300 17,400 19,700Tanks 0 17,000 17,000Heater-Treater 0 20,000 20,000Flowline 0 3,000 3,000Packer 0 5,000 5,000Misc.Equipment 0 15,000 15,000Total Tangibles 49,620 318,675 368,295Total Well Cost 889,855 472,625 1,362,480å Hart Publications,Inc.,Denver;reproduced by permission.6Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration2.History of Technology Development and DeploymentHalting StepsThe modern concept of non-straight line,relatively short-radius drilling,dates back at least to September8,1891,when the first U.S.patent for the use of flexible shafts to rotate drilling bits was issued to JohnSmalley Campbell (Patent Number 459,152).While the prime application described in the patent wasdental,the patent also carefully covered use of his flexible shafts at much larger and heavier physicalscales"...such,for example,as those used in engineer's shops for drilling holes in boiler-plates or otherlike heavy work.The flexible shafts or cables ordinarily employed are not capable of being bent to andworking at a curve of very short radius..."The first recorded true horizontal oil well,drilled near Texon,Texas,was completed in 1929.9Anotherwas drilled in 1944 in the Franklin Heavy Oil Field,Venango County,Pennsylvania,at a depth of 500feet.10China tried horizontal drilling as early as 1957,and later the Soviet Union tried the technique.11Generally,however,little practical application occurred until the early 1980's,by which time the adventof improved downhole drilling motors and the invention of other necessary supporting equipment,materials,and technologies,particularly downhole telemetry equipment,had brought some kinds ofapplications within the imaginable realm of commercial viability.Early Commercial Horizontal WellsTests,which indicated that commercial horizontal drilling success could be achieved in more than isolatedinstances,were carried out between 1980 and 1983 by the French firm Elf Aquitaine in four horizontalwells drilled in three European fields:the Lacq Superieur Oil Field (2 wells) and the Castera Lou OilField,both located in southwestern France,and the Rospo Mare Oil Field,located offshore Italy in theMediterranean Sea.In the latter instance,output was very considerably enhanced.12Early productionwell drilling using horizontal techniques was subsequently undertaken by British Petroleum in Alaska'sPrudhoe Bay Field,in a successful attempt to minimize unwanted water and gas intrusions into theSadlerochit reservoir.139"The Austin Chalk & Horizontal Drilling,"Popular Horizontal,(January/March 1991),p.30.10A.B.Yost,II,W.K.Overbey,and R.S.Carden,"Drilling a 2000-foot Horizontal Well in the Devonian Shale,"Society ofPetroleum Engineers Paper Number 16681,presented at the SPE 62nd Annual Technical Conference,Dallas,Texas,(September1987) pp.291-301.11"Horizontal Drilling Contributes to North Sea Development Strategies,"Journal of Petroleum Technology,(September,1990),p.1154.12"Horizontal Drilling Contributes to North Sea Development Strategies,"Journal of Petroleum Technology,(September,1990),p.1154.13These are referred to as"water-coning"and"gas-coning"due to the hydraulic profile of these fluids that develops in rocksin the vicinity of an active conventional production well.In such cases,the increased stand-off from the fluid contacts in thereservoir that is provided by a horizontal well bore can improve production rates without inducing coning,as the additionalwellbore length serves to reduce the drawdown for a given production rate.Dave W.Sherrard,Bradley W.Brice,and David G.MacDonald,"Application of Horizontal Wells at Prudhoe Bay,"Journal of Petroleum Technology,(November 1987),pp.1417-1425.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration7The Recent Growth of Commercial Horizontal DrillingTaking a cue from these initial successes,horizontal drilling has been undertaken with increasingfrequency by more and more operators.They and the drilling and service firms that support them haveexpanded application of the technology to many additional types of geological and reservoir engineeringfactor-related drilling objectives.Domestic horizontal wells have now been planned and completed in atleast 57 counties or offshore areas located in or off 20 States.Horizontal drilling in the United States has thus far been focused almost entirely on crude oil applications.In 1990,worldwide,more than 1,000 horizontal wells were drilled.Some 850 of them were targeted atTexas'Upper Cretaceous Austin Chalk Formation alone.14Less than 1 percent of the domestic horizontalwells drilled were completed for gas,as compared to 45.3 percent of all successful wells (oil plus gas)drilled.15Of the 54.7 percent of all successful wells that were completed for oil,6.2 percent werehorizontal wells.Market penetration of the new technology has had a noticeable impact on the drillingmarket and on the production of crude oil in certain regions.For example,in mid-August of 1990,crudeoil production from horizontal wells in Texas had reached a rate of over 70,000 barrels per day.16Types of Horizontal Wells and Their Application FavorabilitiesFor classification purposes that are related both to the involved technologies and to differential applicationfavorabilities,petroleum engineers have developed a categorization of horizontal wells according to theradius of the arc described by the wellbore as it passes fromthe vertical to the horizontal.Wells with arcsof 3 to 40 foot radius are defined as short-radius horizontal wells,with those having 1 to 2 foot radii beingconsidered"ultrashort-radius"wells.Some of these short-radius horizontal wells may have angle increasesfromthe vertical,called"build rates",of as much as 3 degrees per foot drilled.Medium-radius wells havearcs of 200 to 1,000 foot radius (that is,build rates of 8 to 30 degrees per 100 feet drilled),while long-radius wells have arcs of 1,000 to 2,500 feet (with build rates up to 6 degrees per 100 feet).17Therequired horizontal displacement,the required length of the horizontal section,the position of the kickoffpoint,and completion constraints are generally considered when selecting a radius of curvature.Most newwells are drilled with longer radii,while recompletions of existing wells most often employ medium orshort radii.Longer radii tend to be conducive to the development of longer horizontal sections and toeasier completion for production.Short-Radius Horizontal WellsShort-radius horizontal wells are commonly used when re-entering existing vertical wells in order to usethe latter as the physical base for the drilling of add-on arc and horizontal hole sections.The steel casing(lining) of an old vertical well facilitates attainment of a higher departure (or"kick off") angle than canbe had in an uncased hole,so that a short-radius profile can more quickly attain horizontality,and thereby14Steven D.Moore,"Technology for the Coming Decade,"Petroleum Engineer International,(January 1991),p.17.15Finance,Accounting and Statistics Department,American Petroleum Institute,Joint Association Survey on 1990 DrillingCosts,(November 1991),p.4-5.16"The Austin Chalk & Horizontal Drilling,"Popular Horizontal,(January/March 1991),p.30.17For medium- and long-radius:B.A.Shelkholeslami,B.W.Schlottman,F.A.Seidel,and D.M.Button,"Drilling andProduction Aspects of Horizontal Wells in the Austin Chalk,"Journal of Petroleum Technology,(July 1991),SPE Paper Number19825,p.773.;other:S.D.Joshi,Horizontal Well Technology,Pennwell Publishing Company,(1991),Tulsa Oklahoma,pp.13-18;for all:Lynn Watney,"Horizontal Drilling Is Feasible in Kansas,"The American Oil & Gas Reporter,(August 1992),pp.84-86.8Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administrationrapidly reach or remain within a pay zone.The small displacement required to reach a near-horizontalattitude also favors the use of short-radius drilling in small lease blocks.A need to avoid extended drillingin a difficult overlying formation also favors use of a short-radius well that kicks off near the bottom of,or below,the difficult formation.Short-radius horizontal drilling also has certain economic advantages.These include a lower capital cost,18the fact that the suction head for downhole production pumps issmaller,and that use of an MWD system is frequently not required if long horizontal sections are not tobe drilled.A current drawback to the use of a short-radius horizontal well is that the target formation should besuitable for an open hole or slotted liner completion,since adequate tools do not yet exist to reliably doproducing zone isolation,remedial,or stimulation work in short-radius holes.Also,hole diameter canonly range up to about 6 inches,and the hole cannot be logged since sufficiently small measurement toolsare not yet available.Medium-Radius Horizontal WellsMedium-radius horizontal wells allow the use of larger hole diameters,near-conventional bottom hole(production) assemblies,and more sophisticated and complex completion methods.It is also possible tolog the hole.Albeit that the drilling of medium-radius horizontal wells does require the use of an MWDsystem,which increases drilling cost,19medium-radius holes are perhaps the most popular current option.They can be drilled on leases as small as 20 acres.20One firm,Meridian Oil,Inc.,accounted for 43percent of all medium-radius horizontal wells drilled in 1989 in the United States,according to the Oiland Gas Investor.21Long-Radius Horizontal WellsLong-radius holes can be drilled using either conventional drilling tools and methods,or the newersteerable systems.Long-radius wells,in the form of deviated wells (not,however,deviated to thehorizontal),have been around quite a while.They are not suited to leases of less than 160 acres due totheir low build rates.Attainable Length of Horizontal DisplacementThe attainable horizontal displacement,particularly for medium- and long-radius wells,has grownsignificantly,as operators and the drilling and service contractors have devised,tested,and refined theirprocedures,and as improved equipment has been designed and used.For example,it was found that somerotation of the drill string,while using a downhole motor to turn the bit,itself aids in passage of the drillstring through the arc from vertical to horizontal.It avoids potentially damaging and power consumptivestick-slip behavior when the string contacts the side of the hole.Some operators have also found the use18"Horizontal Drilling and Completions:A Review of Available Technology,"Petroleum Engineer International,(February1991),pp.14-15.19Rainer Jurgens and others,"Horizontal Drilling and Completions:A Review of Available Technology,"Petroleum EngineerInternational,(February 1991),p.18.20Lynn Watney,"Horizontal Drilling Is Feasible in Kansas,"The American Oil & Gas Reporter,(August 1992),p.84.21Sandra Johnson,"Bakken Shale,"Oil and Gas Investor,Vol.9,No.11,(June 1990),p.36.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration9of coiled tubing drill strings in lieu of conventional jointed drill pipe an advantage in extending thehorizontal displacement of the well (about which more is said later on).Routinely achievable horizontaldisplacements have rapidly climbed from 400 to over 8,000 feet.22The Completion of Horizontal Wells for ProductionDrilling and completion methods,including drilling underbalanced,23have been developed or customizedfor horizontal applications to minimize formation damage during drilling and completion.These methodscan be categorized into the areas of"well logging and formation testing,""well cleanup and wellstimulation,""open hole completion,"and"cased completion."Well Logging and Formation TestingWell logs are usually run prior to completion of a well.They continuously record a suite of measurementsalong the length of the hole,and are interpreted to provide a complete record of the lithologies penetratedand their fluid content.Target horizons for completion are usually selected based on the logs.Additionally,formation testers often are used to determine the ability of selected target zones to producefluids into the well,as well as to secure samples of the fluids.Wall Cleanup and Well StimulationMechanical scraping,acid treatment,and other methods may be used to clean the wall of the well borewithin target producing intervals,so that"virgin"formation is exposed in the well.Fractures around thewell bore in those intervals may also be induced or expanded by explosive,chemical,or hydraulic means,in order to increase the effective well radius by increasing the permeability of the formation.For example,in the Giddings Field's Austin Chalk reservoir,where the oil resides in the natural fracturesystem and not in the rock matrix,both horizontal and vertical wells are stimulated successfully bypumping several tens of thousands of barrels of fresh water into the formation using wax beads as adiverter,alternating with stages of 10 to 15 percent hydrochloric acid.This process opens existingfractures,connects some fractures,and dissolves salt crystals in natural fractures.The result is an increasein the drainage area for a well and,therefore,in reserves per well.One company achieved an average22Rob Buitenkamp,Steve Fischer and Jim Reynolds,"Well claims world record for horizontal displacement,"World Oil,(October 1992),p.41.23Normally,conventional vertical wells are drilled at internal wellbore pressures (usually created by adding dense weightingagents such as barite to the drilling fluid) higher than those expected to be encountered in the penetrated formations.This makesit easier to control any gas"kicks"that are encountered and,thus,minimizes the probability of blowouts.One of the completionproblems caused by this procedure is that some drilling fluid is inevitably injected into the pores of the producing interval,reducing its permeability to the well bore,and,therefore,the achievable production flow.Since fluid is exiting to the formation,a"cake"of solids may also coat the hole opposite the formation.Such damage has to be corrected during well completion.However,a 10-foot vertical pay zone is a lot easier and cheaper to clean up than a horizontal interval of several hundred feet.Most horizontal wells are therefore drilled"underbalanced,"that is,the wellbore pressure during drilling is held at a level slightlyless than that in the formations encountered.This allows formation fluids to flow into the wellbore during drilling,keeping theformation clear of drilling fluid.The goal,of course,is to just barely underbalance,so that a serious safety hazard is not createdwhen entering or passing through a gas-bearing interval.10Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administrationreserve addition per well,over a run of 57 horizontal wells,of 74,000 barrels of oil equivalent per fracturetreatment,at a cost of $ 1.69 per barrel added.24Also in the Austin Chalk,in Texas'Pearsall Field,water fracture treatments and other more conventionalstimulation methods yielded inconsistent results.Consequently,closed fracture acidizing was successfullytried.The acid treatment is injected into the formation at pressures insufficient to induce fracturing,andis allowed to remain for some time so that it can etch out the natural fractures in the rock and clean thefracture faces.25Open Hole CompletionOpen hole completions are those in which nothing is done to modify the raw well bore in the targetproducing zone.They can only be attempted in formations which are structurally competent and,therefore,not prone to collapse or the spalling of rock particles fromthe hole wall as produced fluids flowalongside,andwhich will not produce fines along with the fluids that could clog the well or producingequipment.Open hole completion is,of course,the cheapest alternative if one is certain that futureproblems will not occur.Cased CompletionCased completions are more the norm.The installation (setting) of relatively thin-walled casing in thewell bore allows most possible production problems to be avoided.The casing process consists of hangingthe casing in the hole,cementing it in place,isolating the producing horizon with some combination ofcement plugs and tools called packers,perforating the casing and any cement opposite the desiredproducing interval and,perhaps,installing a production liner.Aspects of each of these are discussedbelow.CasingWell casing consists of thin-walled tubing,usually constructed of steel,that is used to line the drilled hole.The casing supports the wall of the well,checks the caving tendencies of unconsolidated formations,prevents unwanted exchange of fluids between the various penetrated formations,excludes the inflow offluids and fines from all but the target producing intervals,and provides the mounting base for surfacewell control equipment.Normally,the casing is  inch or more smaller in diameter than the drilled hole.CementingCased wells are nearly always cemented (i.e.,cement is pumped down through the well into the annularspace between the casing and the hole wall).The cement serves to mechanically stabilize the casing stringwithin the hole and seals off water flows from the adjacent formations.24Pat Chisholm and Kevin Dunn,"Halliburton Chalk Stimulation - Stimulating the Austin Chalk,"Popular Horizontal,(April/June 1991),p.24;and Louise Durham,"Horizontal action heats up in Louisiana,"World Oil,(July 1992),pp.39-42.25Chisholm & Dunn,op.cit.,p.28.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration11PackersPackers are devices that can be placed at a desired position within a well and then be expanded indiameter to seal off the well bore or casing string at that point.Some are designed to allow passage ofsmaller diameter production tubing through them.Perforations and LinersIn a fully cased well,there are two ways to access target producing intervals.The first is to perforate thecasing and any cement opposite the selected producing horizons,utilizing a perforating gun which containsa number of shaped explosive charges.The second is to set an uncemented well liner at the selectedhorizon in lieu of casing.Liners are pre-perforated,or have slots cut in their sides,or have screen inserts.These openings may or may not be backed up by screens and/or immobile granular packings.The screensand packings serve to keep rock particles from entering the well along with the produced fluids,therebyavoiding contamination of the product stream and possible clogging of,or damage to,the well andproducing equipment.Well completion plans for long radius horizontal wells are determined mainly by the length of thehorizontal section;they differ little from conventional well completions in terms of difficulty.But formedium radius horizontal well completions,problems begin"with running casing and increase with buildrate because conventional equipment no longer works."26Some of the problems reported in the literatureinclude:Production rate-sensitive sand coproduction,which occurs when formation stresses exceedformation strength.27Restriction of fluid flow by prepacked production screens,due to the average pore throatsof commonly used gravels being quite small (from 50 to 100 microns),or failure of theplastic coat on the gravel due to flexing of the gravel pack as the screen is lowered intothe production zone,or failure of the plastic coated gravel filling due to mud acidaction.28The need for completion fluids with special properties relative to viscosity and shearthinning effects.The viscosity-density enhancers commonly used for vertical completions,such as barite and bentonite,cause more than acceptable formation damage in horizontalapplications.Something like a sized-salt polymer system is needed instead.29Centralization of pipe in cased horizontal completions is"difficult to achieve,and mostdesigns are not strong enough to get to the bottom and still work.Medium radiushorizontal completions also present the conflicting objectives of successfully clearing the26T.E.Zaleski,Jr.,and Edward Spatz,"Horizontal completions challenge for industry,"Oil & Gas Journal,(May 2,1988),p.58.27M.R.Islam and A.E.George,"Sand Control in Horizontal Wells in Heavy-Oil Reservoirs,"Journal of PetroleumTechnology,(July 1991),p.844.28Derry D.Sparlin and Raymond W.Hagen,Jr.,"Prepacked screens in high angle and horizontal well completions,"Offshore,(April 1991),pp.57-58.)29Jay Dobson and T.C.Mondshine,"Unique Completion Fluid Suits Horizontal Wells,"Petroleum Engineer International,(September 1990),p.42.12Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administrationcurved portion of the well and having a high density of centralizers on the casingstring."30Current Domestic ApplicationsAs noted previously,horizontal drilling is usually undertaken to achieve important technical objectivesrelated to specific characteristics of a target reservoir.These characteristics typically involve:· the reservoir rock's permeability,which is its capacity to conduct fluid flow through theinterconnections of its pore spaces (termed its"matrix permeability"),or through fractures (its"fracture permeability"),and/or· the expected propensity of the reservoir to develop water or gas influxes deleterious toproduction,either fromother parts of the reservoir or fromadjacent rocks,as production takesplace (an event called"coning").Due to its higher cost,horizontal drilling is currently restricted to situations where these characteristicsindicate that vertical wells would not be as financially successful.In an oil reservoir which has goodmatrix permeability in all directions,no gas cap and no water drive,drilling of horizontal wells wouldlikely be financial folly,since a vertical well programcould achieve a similar recovery of oil at lower cost.But when low matrix permeability exists in the reservoir rock (especially in the horizontal plane),or whenconing of gas or water can be expected to interfere with full recovery,horizontal drilling becomes afinancially viable or even preferred current option.Most existing domestic applications of horizontaldrilling reflect this"philosophy of application.""Chalk Formations"By far the most intensive domestic application of horizontal drilling has been in a few Texas oil fields inwhich the Upper Cretaceous Austin Chalk Formation is the reservoir rock.At year-end 1990,some 85percent of all domestic horizontal wells had been drilled to the Austin.The formation is a massive,oil-bearing limestone that,in some locations,is extensively vertically fractured.Most of the productivepermeability in the formation is fracture permeability,rather than matrix permeability.As a consequence,horizontal wells drilled to intersect several vertical fractures at an approximate right angle have typicallydemonstrated much larger initial production rates than were provided by previously drilled vertical wells.The latter,of course,at best intersected only one vertical fracture.Production from Austin Chalk horizontal wells in the Pearsall Field has been responsible for the recentincrease of oil production experienced in Texas Railroad Commission District 1.A number of the wellstested at flows of over 1,000 barrels per day,a relatively unusual event in the modern day lower 48 Statesonshore.For example,the Winn Exploration Co.10 Leta Glasscock tested at 5,472 barrels of oil per dayaccompanied by 2,368,000 cubic feet per day of gas.31Typically,in Pearsall Field,the Austin Chalkhas produced better after acid treatment of the producing intervals.3230Zaleski and Spatz,op.cit.,p.62.31"TEXAS,"Oil & Gas Journal,(January 1,1990),p.84.32Chisholm and Dunn,op.cit.,p.31.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration13Another Austin Chalk field,the Giddings Field,has served as a comparative testing arena for twoapplication modes of horizontal drilling.In the first,a lateral displacement of about 300 feet was usedto reach a comparatively small area of faulted and fractured rock,with the small horizontal reach in thetarget formation being little beyond that achievable with a vertical well.In the second,the longestpossible horizontal reach was drilled in the pay zone,perpendicular to fracture direction.An eight-wellAmoco Production Company program showed an increase of productivity with increased length ofhorizontal displacement,relative to offsetting vertical wells.The productivity ratio (quantity obtained fromthe horizontal hole relative to quantity obtained from the offset vertical hole) measured at 500 feet of HDwas 2 ,whereas at 2,000 feet of HD it was 7.0.33 34No stimulations were performed on any of theAmoco wells,as"the direct connection between the horizontal wellbore and natural fracture systems [was]sufficient to yield expected producing rates."35Amoco also noted that"it was both cost-effective andoperationally attractive to install pumping equipment immediately after drilling,because it eliminated thecost of swabbing or gas injection to kick the well off."Others have completed their Giddings Field wells using fracture treatments.Chisholm and Dunn notedthat,in their experience,"In general,the Giddings side of the Chalk can best be stimulated with a simplehigh rate,high volume water fracture."36"A water fracturing treatment is the process of pumping largevolumes of fresh water at high rates into the wellbore,alternating with stages of 10 to 15 percent HCl(hydrochloric acid).Hydraulic fractures created by water fracturing tie smaller fractures to each other andto major fracture systems."37Table 2.Giddings Field Horizontal Well Production,6-Month AverageCountyItem Lee Burleson Fayette Brazos TotalOil production,barrels 88,694 2,386,098 498,681 51,100 3,024,573Gas production,thousand cubic feet 75,258 8,236,483 1,440,941 85,025 9,944,707Production,barrels of oil equivalent 95,920 3,209,746 653,775 59,603 4,019,044Number of wells 7 71 12 1 91Production,barrels of oil equivalent per well 13,574 45,208 54,481 59,603 44,165Note:Washington County deleted from table as values are all equal to zero.Study includes only wells with 6 monthsof production history drilled after August 1989.Source:William T.Maloy,"Horizontal wells up odds for profit in Giddings Austin Chalk,"Oil & Gas Journal,(February 17,1992),p.68.Reproduced by permission.33B.A.Shelkholeslami and others,op.cit.,p.778.34Shelkholeslami,et al.,op.cit.,p.778.35Shelkholeslami,et.al.,op.cit.,p.778.36Chisholm and Dunn,op.cit.,p.31.37Chisholm and Dunn,op.cit.,p.24.14Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information AdministrationHorizontal drilling in the Giddings Field has not only significantly improved average well recoveries,ithas more than offset the increased drilling costs.A study of 91 horizontal wells,all drilled after August1989,and all with at least 6 months of production history,showed an average 195,000 barrels of oilequivalent recovery over the economic well life.Three-fourths of this amount is obtained in the first 3years.(Table 2).The study indicated that an average Giddings Field Austin Chalk horizontal well wouldreturn an after-tax investment (discounted at 10 percent) of 1.6:1,would have a net present value of$650,923,and would pay out its cost in 1.1 years.38Additional"chalk"formations in which horizontal drilling is being attempted include the Annona andSaratoga members of the Upper Cretaceous Selma Group in Louisiana,the Lower Cretaceous Buda andGeorgetown Formations (both Washita Group) in Texas,and the Upper Cretaceous Niobrara Formationin Colorado and Wyoming.In the latter instance,the technique is being tested amidst much tougherdrilling conditions than prevail in the Austin Chalk.Drilling problems encountered in the northeasternDenver Basin's Silo Field include sloughing of the overlying Pierre Shale,problems in the Niobrara itselfwith over- and underpressured intervals,and karst,vuggy,or fractured zones that can cause loss of drillingfluid circulation or hole stability problems.39Other ApplicationsBeyond the fractured,low matrix permeability class of reservoirs exemplified by the various chalkformations,there are numerous other geologic situations in which horizontal drilling is being applied,albeit with less frequency.Early applications at Prudhoe Bay Field to avoid or minimize either water orgas coning have already been mentioned;many similar applications have since been executed elsewherefor the same purpose.Several specific examples of types of applications which appear to form the bulkof additional domestic horizontal drilling to date are discussed hereafter.Source Rock ApplicationsOne type of application attempts to produce oil that has not yet migrated to a conventional trap,butinstead remains in the porosity of the source rock unit in which it was generated.A prime example is theMississippian Bakken Formation of North Dakota and Montana,which,in generationally mature areas,is an oil-wet shale believed to contain several billion barrels of oil-in-place.Meridian Oil,Inc.,indicated that its Bakken horizontal drilling program had added net reserves of morethan 16.6 million barrels of oil equivalent by March 1992.40Meridian's program followed a very clearlearning curve.The first 10 wells had an after-tax rate of return of 30.6 percent,which climbed to 44.2percent for the second 10 wells,and again to 66.6 percent for the third 10 wells.Pacific Enterprises OilCo.indicated that,compared to vertical wells on 160 acre spacings,its Bakken horizontal wells,spacedat 320 acres,provided a 40-percent greater return on investment.41Note that horizontal wells generallyrequire larger well spacings than conventional vertical wells in order to avoid drainage of neighboringleases and fluid communication with neighboring wells;in at least one case,fluids injected at highpressure into a horizontal well during its completion to fracture the surrounding rock were produced by38WilliamT.Maloy,"Horizontal wells up odds for profit in Giddings Austin Chalk,"Oil &Gas Journal,(February 17,1992),pp.67-70.39G.Alan Petzet,"Horizontal Niobrara play proceeding with caution,"Oil & Gas Journal,(November 11,1991),p.68.40M.G.Whitmire,"Fractured zones draw horizontal technology to Marietta basin,"Oil & Gas Journal,(March 30,1992),p.78.41Sandra Johnson,"Bakken Shale,"Western Oil World,(June 1990),pp.31-45.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration15a nearby existing well.In North Dakota,oil production from horizontal completions has risen steadilyuntil this year,as seen in Table 3.Stratigraphic Trap ApplicationsOn the basis of first principles,it would seem that horizontal drilling would be the method of choice forthe drilling of certain kinds of stratigraphic traps such as porosity pinchouts and reefs.Yet,remarkablyfew domestic examples of pure stratigraphic trap horizontal drilling have been reported in the tradeliterature.Some effort (10 wells over the past 5 years) has been reported to develop Silurian Niagaranreef structures in the Michigan Basin.The first such well,drilled by Trendwell,was successful,but hasyet to produce due to high hydrogen sulfide content and a lack of suitable treatment facilities.42A recentsuccess,the Conoco 2-18 HD1 Lovette,was drilled in Vevay Township,Ingham County.It had an HDof 1,100 feet,and produced 149 barrels per day of crude oil accompanied by 77,000 cubic feet per dayof natural gas.The relatively low production rates of the existing Niagaran horizontal wells have notenhanced the attractiveness of horizontal drilling for these kinds of targets.43Eventual drilling of a"flush"(highly productive) well or two would,no doubt,rapidly alter that situation.Heterogeneous ReservoirsAnother type of application seeks to overcome problems caused by reservoir heterogeneity.For example,Amoco reentered an existing Ryckman Creek Field (Wyoming) well and drilled a lateral about 500 feetinto the Upper Triassic Nugget Formation,with multiple objectives of seeking to open more pay zone,penetrate more"sweet"spots (so called because they are the more productive areas of the heterogeneousreservoir),attain better maintenance of reservoir pressure,and reduce water and gas coning.44Coalbed Methane ProductionShort and medium radius horizontal drilling techniques for coalbed methane recovery have beensuccessfully demonstrated.Short radius technique was used by Gas Resources Institute (GRI)/ResourceEnterprises,Inc.in the No.3 Deep Seam gas well drilled into the Cameo"D"Coal Seam,Piceance Basin,Colorado.The Department of Energy and GRI used medium radius technique successfully at their RockyMountain No.1 site in the Hanna Basin,Wyoming,targeting the Hanna coal seam at 363 foot depth.Subsequently,commercial horizontal wells have been drilled into the Fruitland Coal of NewMexico's SanJuan Basin by several firms.45Meridian Oil,Inc.,brought in one such well that produced at a rate of7 million cubic feet per day,as opposed to the average conventional well rate of 1.05 million cubic feetper day.4642Scott Ballenger,Michigan Oil & Gas News,personal communication,(June 24,1992),43Ballenger,op.cit.44Russ Rountree,"Amoco tests horizontal drilling,"Western Oil World,(September 1987),p.18.45Terry L.Logan,"Horizontal Drainhole Drilling Techniques Used in Rocky Mountains Coal Seams,"1988-Coal-BedMethane,San Juan Basin,Rocky Mountain Association of Geologists,pp.133-141.46"Meridian tests new technology,"Western Oil World,(June 1990),p.13.16Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information AdministrationTable 3.Horizontal Oil Activity and Production,North DakotaTimePeriodDryHolesProducingWellsProduction(Barrels)Percent ofTotal StateProduction1986 0 0 0 0.01987 0 1 10516 <0.11988 0 9 222805 0.61989 3 29 969075 2.61990 12 65 2744317 7.51991 3 45 3706820 10.31992 0 34 3004867 9.1Source:North Dakota Industrial Commission,Oil and Gas Division.Boosting Recovery FactorYet another type of horizontal drilling application attempts to increase the recovery factor (the producedfraction of oil-in-place) experienced in already mature reservoirs.An example occurred in thePennsylvanian Bartlesville sand,a fluvial-dominated deltaic sandstone reservoir characterized by lowpermeability and underlain by a sandstone aquifer.In the Flatrock Field of Osage County,Oklahoma,theBartlesville is located at a depth of 1,400 feet.The field,discovered in 1904,was considered fullydeveloped by 1925,with over 1,000 conventional wells;it has produced over 30 million barrels of oil.The old vertical wells were typically fractured using explosives,which increased initial oil production ratesto economic levels and usually avoided deleterious over-break into the underlying water-bearing unit.However,they also typically developed a large water cut (water as a fraction of all produced liquids) bytheir 12th month of service.It was hoped that a horizontal well would both increase unstimulated initialoil production and reduce total water production.47A well was completed in the 10-foot thickBartlesville Sand at a HD of 1,050 feet.Unstimulated initial oil production was not materially increasedin this instance (on the order of 6 to 9 barrels of oil per day),but the watercut was substantially lessened(after 90 days,from roughly 75 percent for vertical wells,to 14 percent with the horizontal well).Afterexplosive stimulation intended to improve oil production,the watercut increased to high levels becausethe aquifer was unintentionally breached.Fluid and Heat Injection ApplicationsHorizontal drilling technology has also inspired new approaches to the injection of fluids or heat into oilor tar sand reservoirs to enable or improve recovery.One of the more recent technologies is heatedannulus steam drive (HAS drive),now under pilot study by Chevron Canada at Steepbank Field innortheast Alberta,Canada.The process involves circulating steam in an unperforated horizontal tar sandwell.The pilot well has a horizontal section of 1,600 feet and a TD of 2,800 feet.In this instance,thehorizontal well is located below conventional vertical perforated steam injection and production wells.4847John E.Rougeot and Kurt A.Lauterbach,"The Drilling of a Horizontal Well in a Mature Field,"(January 1991),DOE/BC/14458-1,p.1.48"Horizontal Well Used as Heating Coil in Bitumen Recovery,"Petroleum Engineer International,(May 1992),p.15.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration17The Glenn Pool Field,Tulsa County,Oklahoma,is the site of a Department of Energy-funded project toenhance oil production fromfluvial-dominated deltaic reservoirs utilizing reservoir characterization studiesand horizontal water injection technology.49Dramatic oil production gains have been reported in the New Hope Field,Franklin County,Texas,byTexaco Exploration & Production Inc.,utilizing two horizontal injection wells drilled into the LowerCretaceous Pittsburg reservoir.The Pittsburg is a relatively thin,low permeability sandstone.Thehorizontal wells were placed about 8,000 feet deep,lower on the anticlinal structure of the field thanexisting producing wells.Reservoir simulations were used to select the locations for the wells,which areconsidered true line drive waterfloods,and were completed open hole.Since introduction of the horizontalinjection wells,production per producing well has increased from about 100 to 400 barrels of oil per dayvia submersible pumping units,the highest production rates in the history of the field.50Costs of thedevelopment are estimated at $2 to $3 per barrel of incremental reserves added.Company officialsestimate that the productive life of the New Hope shallow unit has been extended by 10 to 15 years.51Win Energy,Inc.,has reported a plan to drill up to eight horizontal water injection wells into the PermianFlippen Limestone in the combined Bullard Unit/Propst-Anson Fields,Jones County,Texas.The companyestimates that 1 million barrels of additional oil production will come from the combined fields,whichare believed to have had original oil-in-place of 6 million barrels and a present recovery factor of 32percent.The wells will be located at a relatively shallow 2,540 feet and will be offset horizontally about1,500 feet.52Cyclic steaminjection through multiple ultrashort radius horizontal radials has been tested in a Departmentof Energy-sponsored project at the Midway-Sunset Field,California.The field has a history of successfulthermal operations and is California's second largest current producer.A set of eight radials was drilledinto a cold zone within the 400-foot thick Upper Miocene Potter C reservoir interval,which is a fairlyclean quartz sandstone containing about 10 percent shale,located at a depth of 884 feet.Temperaturelogging in an observation well located 50 feet from the end of one of the radials showed a substantialtemperature increase in the 800- to 875-foot interval,demonstrating effective containment of steam in thetarget interval.Decrease of steam override effects near vertical well bores was also a goal of the well,one which so far has been attained.Production from the well started out very low in the first week andthen increased over the next 3 weeks to a peak of 60 barrels of oil per day with a 30 percent water cut.Production stayed strong from mid-July 1990,through the first week of October.53Multiple ObjectivesIn some instances,it has been possible to target reservoirs exhibiting more than one characteristicfavorable to the application of horizontal drilling technology.A good example is the Grassy Creek TrailField located in Emery and Carbon counties,Utah,which produces oil fromseveral members of the LowerTriassic Moenkopi Formation.The field is located on a small,low dip structural nose at the north endof the San Rafael Swell.Lithologies in the producing zones are predominately siltstones and shales thathave low matrix porosity and permeability.The oil appears to have been sourced within the Moenkopi49"DOE chooses 14 EOR projects for backing,"Oil & Gas Journal,(April 27,1992),p.28.50"Horizontal Wells Inject New Life Into Mature Field,"Petroleum Engineer International,(April 1992),pp.49-50.51"Texaco completes horizontal injector in Southeast Texas oil field,"Oil & Gas Journal,(February 24,1992),p.44.52"Horizontal waterflood scheduled in Texas carbonate reservoir,"Oil & Gas Journal,(September 2,1991),p.34.53Wade Dickinson,Eric Dickinson,Herman Dykstra,and John M.Nees,"Horizontal radials enhance oil production from athermal project,"Oil & Gas Journal,(May 4,1992),pp.116-124.18Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administrationitself,and the producing zones are vertically fractured.Depth to production is relatively shallow,on theorder of 3,900 feet.The field was discovered by Cities Service Oil Company in 1953,after which four development wellswere completed.The five vertical wells produced about 141,000 barrels of oil between 1961 and 1976.Skyline Oil Company began a second development program in 1982 that applied multiple short (severalhundred foot) lateral borehole completions executed using the Texas Eastern Drilling Systems,Inc.(Tedsi)technology for short-radius horizontal wells.Sixteen were drilled,of which 13 had delivered productionof 358,817 barrels of oil through 1987 or,in a period of 6 years,2 times the amount delivered by the5 conventional wells during 16 years.Virtually all of the horizontal production came from 10 of thewells,in which 29 of 39 laterals drilled into different members of the Moenkopi Formation wereproductive.Vertical fractures encountered by the laterals were found to range from to 1 inch in widthat interval spacings of from 100 to 200 feet.It is believed that the production curves from the GrassyTrail Creek Field wells showed the influence of two different producing mechanisms.The first washyperbolic decline resulting from rapid gas expansion in those fractures which were in direct contact withthe borehole,while the second was exponential decline resulting from the gravity drainage of fluidentering fractures from the rock matrix at some distance from the borehole.5454Gary C.Mitchell,Fred E.Rugg,and John C.Byers,"The Moenkopi:horizontal drilling objective in East Central Utah,"Oil & Gas Journal,(September 25,1989),pp.120-124.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration193.New DevelopmentsExpected Growth of Horizontal DrillingVirtually all relevant trade journals have carried articles over the past 5 years expressing considerableoptimism as to the business growth prospects of horizontal drilling.So far,these predictions appear tobe valid.A close student of the subject,David Yard,estimated in January 1990 that horizontalcompletions would escalate by 230 percent annually,and that more than 2,000 successful completionscould be expected in 1992.He also expected lifting costs to fall into the $4- to $6-per-barrel range.55Coiled Tubing and Horizontal DrillingInitially tested for petroleum industry applications in the 1960's,56coiled tubing technology has been usedfor some time to perform conventional well workovers (maintenance and remedial work).Its use to doinitial drilling and completion work,particularly in horizontal holes,is a phenomenon of the late 1980'sand the present.Unlike standard drill pipe,which comes in 30-foot lengths equipped with threadedconnectors at each end,and is stored in 3-section,90-foot-long joints on the drilling or workover rig's piperack,coiled tubing is a continuous length of pipe that is stored wrapped around a large reel,in much thesame fashion as thick electrical cable is stored and shipped.In operation,the tubing is straightened offthe storage reel and led over a curved guide to and through a motorized injector head mounted atop thewell control equipment stack,and thence through the control stack into the well.Tools are attached tothe downhole end;wire cables can also be passed,and fluids circulated,through the tubing.The tubing's wall thickness,on the order of 0.05 to 0.2 inches,is considerably less than that of drill pipe,which ranges from about 0.2 inches to about 0.5 inches.Its diameter is also less,on the order of  to2 inches,whereas drill pipe ranges from about 2 inches to about 5 inches in diameter.The smallerdimensionality,as well as the use of different alloys,renders the tubing much more flexible than standarddrill pipe,at the expense of increased fragility and decreased load handling capabilities in bothcompression and tension.In horizontal use,the sliding friction increases as the horizontal displacementincreases,adding to the load in both compression and tension.As noted by Spreux,"The weak link inthe system is its relative fragility,rendering it incapable of pushing heavy tools over great distances."57Particularly to be avoided are the development of local ovalities ("out of round"spots) and the placementof excessive axial tension on the pipe,which can produce"necking"(a local reduction of diameter),bothof which reduce its collapse strength.At the opposite extreme,excessive compression,the combinedresult of the downward force applied by the injector head and the upward force caused by frictional dragof the downhole tools,will cause buckling of the tubing,which can ultimately deform into a helix thatis in contact with the wellbore along the entire length of the tubing.At that point,friction becomes sogreat that no further downhole progress can be made.Thus,tighter controls on operating conditions andhandling methods are required in coiled tubing applications than are normally applied when usingconventional drill strings.55"Reservoir Engineering is Key to Horizontal Drilling,"Petroleum Engineer International,(March 1990),p.49.56Alexander Sas-Jaworsky,II,"Coiled tubing...operations and services,Part 1-The evolution of coiled tubing equipment,"World Oil,(November 1991),pp.41-44,46-47.57A.M.Spreux,A.Louis,and M.Rocca,"Logging Horizontal Wells:Field Practice for Various Techniques,"Journal ofPetroleum Technology,(October 1988),pp.1352-1354.20Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information AdministrationTo relieve compression problems in the use of coiled tubing to push tools in horizontal well maintenanceoperations,Statoil and others58are combining two variants of the pumpdown technology of conventionalwireline operations with coiled tubing use:coiled-tubing assisted pumpdown (CTP) and pump-assistedcoiled tubing.In coiled-tubing assisted pumpdown,an independent (not connected to the surface) coiledtubing segment is run into the well using pump locomotives at either segment end.The lower endlocomotive is"floating"so that the tubing can be pushed through it even farther into the well.A hollowlocomotive at the upper end is used to drive the entire assembly back up the well.In the pump-assistedcoiled tubing approach,the tubing runs in the conventional manner all the way from the surface,wherethe tubing is telescoped through a seal adapter.The fluid displaced by the telescoping coiled tubing goeseither into the external annulus or into the formation.A clear advantage of this approach is that loggingtool cable can also be run.Offsetting the consequences of the relative frailty of coiled tubing is the fact that a coiled tubing unit isoften less expensive to operate than a conventional drilling rig,for a number of reasons.A coiled tubingunit is quicker to rig up on-site."Run in"and"pull out"rates can exceed 170 feet per minute,so whennumerous"trips"(removals and reinsertions of the drill string from the hole) are required to replacedownhole motors or tools,or to retrieve cut core,use of a coiled tubing unit significantly reduces totalon-site time and,therefore,cost.Another favorable factor is that fluid circulation can be maintained atall times since there is no need to break apart joints of pipe when"tripping."A bonus,from both costand environmental viewpoints,is that a coiled tubing unit typically has a reduced"footprint"(disturbeddrill site area) and is less noisy relative to a conventional drilling rig.Finally,operations performed bythe use of coiled tubing technology are often less damaging to the potential producing formation than ifperformed using conventional drilling and completion methods.59Application of coiled tubing technology in the drilling,completion,and servicing of horizontal wells hasbeen growing both absolutely and with respect to the range of jobs performed.The lower costs andreduced environmental impacts of coiled tubing technology have contributed to this growth.Directionaldrilling of wells with HD's of over 1,500 feet has been accomplished using coiled tubing.Drill-out ofblockages in existing wells using positive displacement mud motors mounted to coiled tubing have alsoworked,with the advantage that horizontal wells can be entered via coiled tubing without the priorremoval of production tubing or liners.60Coiled tubing is also used in horizontal wells to insert andmanipulate flow control equipment that regulates reservoir drainage,as well as in the traditional wellworkover application,the precision placement downhole of various fluid treatments such as cement slurriesand acid gels.The success of coiled tubing in this use is based on its ability to be pushed into ahorizontal well bore (as compared to a conventional wireline) to a precise position,its ability to work inflowing wells,and its ability to operate hydraulically actuated tools.61Over time,it is to be expected that lighter but nevertheless robust tools will be developed,extending thecapabilities of the technology.In both horizontal and conventional applications,particularly the former,its use is expected to continue to grow rapidly.58Roger J.Tailby,"Pumpdown assistance extends coiled tubing reach,"World Oil,(July 1992),pp.55-61.59Vance Norton,Fred Edens,Glenn Coker,and George King,"Large diameter coiled tubing completions decrease risk offormation damage,"Oil & Gas Journal,(July 20,1992),pp.111-113.60M.Wasson,F.Pittard,and L.Robb,"Horizontal Workover With Coiled Tubing and Motors,"Petroleum EngineerInternational (June 1991),pp.40,42.61Cameron White and Mark Hopman,"Controlling flow in horizontal wells,"World Oil,(November 1991),pp.73-80.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration21Slim Hole Horizontal DrillingA recent trend that undoubtedly will be adapted to both deviated and horizontal applications over the nextfew years is increased use of what is called"slimhole"drilling.Most conventional oil and gas wells havebeen,and continue to be,drilled at a successively smaller series of diameters as depth increases,such thatthe bottom hole segment is on the order of 6 inches or larger,and the higher segments telescope upwardin diameter between the bottom hole and final surface diameters.The surface casing segment may be asmuch as 24 inches in diameter.Slim hole drilling,long practiced in the mining industry to do explorationwork and obtain rock samples,simply reduces the diameter of each segment substantially.The use of slim hole drilling in the oil and gas industry has been made possible by the development ofmaterials and technologies that allow drilling,completion,and production operations within the boundsof the smaller involved diameters.As more instruments and tools are designed and built to accommodatethe smaller diameters of slim holes,there will occur a perfectly natural extension of slim hole drilling todeviated and horizontal drilling operations,due to its principal advantage:reduced cost.For example,steel is priced by the ton and 1,000 feet of casing for 12inch hole weighs 59 tons whilethe equivalent length of 8 inch hole casing weighs only 29 tons.62Lower costs similarly result formany other items such as drill pipe,drill bits,fuel costs,mud chemicals,cement,and cuttings cleaningand disposal.Beyond that,the overall size of the necessary drilling rig,its hook (lifting) capacity,andits footprint can all be lowered by scaling down the hole diameter,although typically there is a loss oftorque transmission capability as diameter is reduced,requiring compensatory use of higher rotation ratesthan are commonly used in conventional drilling.Finally,time to TD is usually reduced,as a smallerdiameter hole is usually much quicker to drill,all other factors being the same.The Drilling of Multiple LateralsYet another trend is increased frequency of the drilling of multiple laterals from the initial vertical sectionof a hole.There are many instances in which two horizontal laterals have been successfully drilled andcompleted,running in opposite directions from the kickoff point.There are also instances (one was citedearlier) in which several short,short-radius laterals have been drilled in fan or radial patterns froma singleinitial vertical hole section.It would appear that,particularly in inhomogeneous reservoir situations,expansion to the drilling of multiple laterals with longer lengths,larger radii,and larger diameters is notchronologically far off.A"Fire and Forget"Drilling SystemSeveral firms are nowdeveloping directional drilling control systems that are supervised by either surface-located or on-board computers.63The computer continuously monitors MWD sensors for downholeconditions,operating parameters,and hole azimuth and inclination.It then feeds back continuous steeringadjustments to the downhole assembly.The results,in tests to date,have been smoothly transient holes62"Smaller Top Hole Equates to Lower Drilling Costs,"Petroleum Engineer International,(September 1992),p.17.63An example is the Automated Guidance System (AGS) under development by Cambridge Radiation Technology,Ltd.(United Kingdom)."Intelligent Tool Automates Directional Drilling,"Petroleum Engineer International,(September 1992),p.13.22Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administrationconforming much more closely to original engineering plans than would have been achieved by the useof standard,human-applied incremental adjustment techniques.Ultimately,full development of such systems may well result in the drilling of deviated and horizontalwells in a"fire and forget"mode,wherein the hole design is loaded into the computer,the drilling processis initiated,and human intervention (other than to perform"maintenance work"such as bit changes andthe like) is thereafter unneeded until TD is reached at exactly the intended location.Such a systemshouldprovide several economic advantages such as less necessity for tripping,higher penetration rates,and lowerpersonnel costs.Gas Research WellsAs noted early on,most domestic horizontal wells have thus far been drilled in search of,or to produce,crude oil.There is no physical reason why they should not also be targeted for natural gas.To promotetheir wider use in this application,two horizontal wells,partially funded by the Department of Energythrough its Morgantown Energy Technology Center (METC),have been drilled for research anddemonstration purposes in West Virginia and Colorado,as part of a program to develop methods toincrease production from impermeable (tight) formations.The West Virginia well,the RecoveryEfficiency Test well,was completed for Devonian shale gas in December 1986.It was the first horizontalwell that used air as the bit cooling and chip removal medium.The pay zone of this well was the UpperDevonian Huron Shale;productive thickness of the lowest section of the Huron in this area is 40 to 60feet.64The well was drilled in a direction orthogonal to the primary natural fracture orientation toimprove the efficiency of natural gas extraction from the shale.6564J.R.Duda,S.P.Salamy,Khashayar Aminian,and Samuel Ameri,"Pressure Analysis of an Unstimulated Horizontal WellWith Type Curves,"Journal of Petroleum Technology,(August 1991),p.988.65J.R.Duda,S.P.Salamy,Khashayar Aminian,and Samuel Ameri,"Pressure Analysis of an Unstimulated Horizontal WellWith Type Curves,"Journal of Petroleum Technology,(August 1991),p.988.Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration234.SummaryThe technology of horizontal drilling has solidly moved into the arsenal of the oil industry over the past10 to 12 years.A particular synergismof developments among equipment,techniques,and economically-driven efficiency requirements has caused widespread consideration and testing of this technology.Inmany basins and reservoirs,properly applied horizontal drilling technology has demonstrated anincremental advantage over vertical wells.24Drilling Sideways -- A Review of Horizontal Well Technology and Its Domestic ApplicationEnergy Information Administration